A significant problem associated with downhole drilling (e.g., drilling wells for hydrocarbons production) is the loss of downhole circulation fluid (sometimes referred to as “bulk fluid”) into the formation. This phenomenon is commonly called lost circulation. Lost circulation may occur when the differential pressure of the hydrostatic column is greater than formation pressure.
The need has increased to drill across zones that are not only low pressure, but highly fractured or permeable. The exposure of numerous fractures or openings having low formation pressures has increased the problem of lost circulation and formation invasion. The openings in the formation are able to accept and store drilling fluid so that little to none is returned to the surface for recirculation. This loss of fluid can become an expensive and dangerous problem. Lost circulation can lead to hole instability, stuck drill pipe, and loss of well control. At the least, it can halt drilling operations and requires expensive replacement of the lost fluid.
In addition to the fluid volume being lost, expensive lost circulation materials (LCM) are required in order to prevent continuous loss. LCM materials have heretofore comprised fibrous, granular, or flake materials such as cane fibers, wood fibers, cottonseed hulls, nut hulls, mica, cellophane, and many other materials. These LCM materials are added to the fluid system so that they may be carried into the loss zone and lodge in and around the formation. These LCM materials are typically added to the zone in the form of a pill. Pills are well known in the art and are typically small amounts of fluid relative to the bulk circulating fluid, i.e., fluid circulating within the drillstring and annulus. Pills are spotted or circulated in a wellbore and are prepared for a variety of functions including enhancement of the properties of the bulk fluid, performance of a specific task that the bulk fluid cannot perform, or a combination thereof.
LCM materials described above have several problems. They can be damaging to the formation zones. Also, solids (drill cuttings and the like) present in the LCM fluids cannot be removed with any of the known or available equipment, i.e., shaker screens, cyclones, etc., at least not in an economical manner. Thus, the fluids generally have to bypass the solids removal mechanisms. Further, the current LCM materials do not plug rapidly in many instances and have to be circulated numerous times through a fractured zone. This results in continuous increase of solids buildup and ultimately a fluid system containing a high concentration of solids.
Another type of lost circulation control technique has been developed in part due to the problems mentioned above and in part due to the increase in underbalanced drilling. Some of the underbalanced drilling techniques include the use of air, mist, and foam drilling fluids. Methods of correcting lost circulation of drilling fluids by aerating the drilling fluids are set forth in U.S. Pat. No. 2,818,230 (Davis) and U.S. Pat. No. 4,155,410 (Jackson). Problems with these fluids include hole cleaning, control of formation fluids, corrosion, and requirements for expensive, often hard to get equipment such as compressors and boosters.
In addition, although some LCM fluids are suitable for plugging the formation fractures and preventing the loss of circulation fluid, in severe loss zones even these fluids have difficulty sealing or plugging the fractures before a significant amount of expensive fluid is lost into the formation. These severe loss zones are typically large fractures in terms of physical opening size and/or in terms of total volume of the fracture. For example, large fractures are considered 10 μm or greater at their smallest opening. Many fractures are several inches or greater and some are sometimes associated with small caves typically called vugs. Even when the smallest opening is near the 10 μm level, the overall fracture may be several inches to several feet tall and several feet to several miles in length. Thus, many of the current LCM drilling fluid compositions may not be able to seal the fracture fast enough to prevent excessive loss.
In light of the deficiencies of the prior LCM materials, there is still a great need for fluids that can rapidly seal formation fractures and/or inhibit the excessive loss of drilling fluids.